Increasing application of electronics for control and management in the automotive field, with widespread involvement of microprocessors, has led to a corresponding requirement for sensors for use in converting mechanical displacement of components during operation into electrical signals proportional thereto. Variable electrical resistance devices, notably potentiometers, are being increasingly utilised in automotive monitoring systems, e.g for monitoring position or displacement of a rotatable throttle in an engine induction manifold. There is also now a requirement for road wheel suspension displacement to be monitored as part of a servo control system for the suspension.
Variable resistance devices commonly comprise a wiper of electrically conductive material supported in a housing and adapted for motion in a linear or rotary sense to traverse a track of electrically resistive material. When operating as a potentiometer, a voltage is applied to terminals connected to the ends of the resistive track and an output voltage, whose magnitude is a function of the position of the wiper along the track, is derived at a terminal connected to the wiper. In order to monitor displacement or position of a mechanical component, the component is mechanically coupled to the wiper and the output voltage derived at the wiper terminal provides a representation of the displacement or position of the component.
Problems arise when a variable resistance device is required to monitor displacement of a component, such as a vehicle road wheel suspension, which is undergoing a large number of operating cycles of varying frequency and amplitude. In such a situation wear occurs in the wiper and/or resistive track leading to failure of the device after an unsatisfactorily short period of time. This wear is further increased by a tendency for the wiper to dither on the track e.g. as a result of mechanical vibration. Longevity of variable resistance devices has been improved by the use of a conductive polymer material for the resistive track since this material, in some measure, possesses self-lubricating properties. Using such a material, a life of twenty million operations of a wiper over the entire length of the resistive track can be achieved. However, even this is insufficient for some applications, such as with vehicle road wheel suspension monitoring systems where a life of ten times longer may be required.
The present invention seeks to overcome this problem by making use of the realisation that continuous monitoring of the output of the variable resistance device is often not required. The invention consequently has for its object the provision of means whereby the wiper and resistive tracks are only brought into full contact with one another when monitoring is required and are arranged to be held out of contact, or with reduced pressure of contact therebetween, at other times.